The present invention relates to the communication system, more specifically, to a variable gain amplifier having improved gain slope characteristic and linearity.
A variable gain amplifier is generally used for maintaining a desired output level in the communication system.
A prior art of a variable gain amplifier may be found in U.S. Pat. No. 5,757,230. The variable gain amplifier of the 230"" patent uses a linearized transconductor in conjunction with an output circuit. The transconductor has a transconductance that varies in response to a first control signal while the output circuit has a transresistance that varies in response to a second control signal. Both control signals are provided by a gain controller circuit portion. The voltage gain of the amplifier is equal to the product of the transconductance and the transresistance. The amplifier can be configured to provide an exponential gain. The amplifier of the U.S. Pat. No. 5,757,230 patent is a complicated device requiring as many as 50 transistors to implement.
Another variable gain amplifier may be found in U.S. Pat. No. 6,201,443. The variable gain amplifier of the U.S. Pat. No. 6,201,443 includes a first gain cell, a second gain cell that is operably connected to the first gain cell, and a current control circuit portion that is operably connected to the first and second gain cells. The first gain cell has a transconductance and preferably comprises a pair of MOSFETs. The second gain cell has a transconductance and also preferably comprises a pair of MOSFETs. The first and second gain cells receive an input signal and produce an output signal that is amplified in proportion to their transconductance. The current control circuit portion controls the amount of current sent to the first and second gain cells and, as such, also controls their transconductance. The transconductance of the first gain cell is designed to be larger by a predetermined amount than the transconductance of the second gain cell for a given amount of current.
The variable gain amplifier has some problems in scope of gain control because gain slope of the amplifier is steep. Further, a non-linear characteristic of the variable gain amplifier is the main reason for deterioration of the performance of the whole circuit of the communication system.
The object of the present invention is to provide a variable gain amplifier whose gain slope characteristic is improved.
Another object of the present invention is to provide a variable gain amplifier whose gain is controlled widely by fixing minimum value of gain.
Another object of the present invention is to provide a variable gain amplifier whose linearity is improved by controlling degeneration impedance value.
Another object of the present invention is to provide a variable gain amplifier that amplifies an input signal with high gain when small signal is inputted, and amplifies an input signal with high linearity when large signal is inputted.